In DTMF (dual tone multifrequency) telephone signaling systems specified frequency sinusoidal signals must be generated when selected touch keys of the telephone are activated. The original and still widely used approach for generating these tones has been the use of conventional LC, discrete component, oscillators. However, oscillators of this type are bulky and expensive.
Integrated circuit tone dialing equipment has been developed in response to the demand for economical and low power devices. However, integrated circuit tone dialers have suffered from a number of serious drawbacks. These include the difficulty of working at the extremely low voltages which can be supplied to telephone receivers and the relative complexity and resulting high cost of tone synthesizing circuits. Conventional tone generation circuits utilize a large number of components which increases the size of the integrated circuit. This in turn increases the cost of the circuit. Therefore there exists a need for a tone synthesizing circuit which has a minimum number of circuit elements in order to reduce the area required for the integrated circuit.